The invention relates to a method of obtaining caustic soda and pure sodium chloride from a sodium chloride-containing liquor also containing sulphate ions and supplied by or recovered from an electrolytic cell.
Caustic soda liquors supplied by an electrolytic cell have a relatively low concentration of caustic soda, in order of 10 to 15% by weight, and a sodium chloride content in the order of 15% by weight. But such liquors generally contain small quantities of other compounds which act as impurities. These impurities include sulphate ions. Sulphate ions interfere with the recovery of the caustic soda and sodium chloride in a pure state.
Various methods have already been proposed for recovering the caustic soda and sodium chloride. One method comprises concentrating the initial liquor by evaporating part of the water contained in it, then cooling the concentrate and finally separating the soda solution from the precipitate thus formed. Various means have been advocated for carrying out such a process. Thus, French Pat. No. 2,158,301 claims the cooling of the concentrated solution by direct contact with a liquid coolant such as liquid butane. However, such a method involves expenditure on a compound which in itself is foreign to the process.
Moreover, the foregoing method of concentrating by evaporation entails high expenditure on thermal energy, since the water contained in the solution has to be evaporated. In similar cases it is known to be economically advantageous to make use of the multiple effect technique; this comprises re-utilizing the steam given off in one evaporator for heating another evaporator, identical with the first and operating at a lower temperature. In this way, a plurality of evaporators may be combined in series. This enables considerable economies to be made in steam consumption and thus in heat. In practice, one is limited by the cost of the installations. This means that a compromise has to be made between capital costs and operating costs. Furthermore, one must take into account the phases involved in carrying out the process.
Thus, in the case of soda liquors from electrolytic cells, the caustic soda and sodium chloride are often required to be separated and extracted separately. The sodium chloride may be collected in the form of a pure salt or a salt containing sodium sulphate. However, the salt containing sulphate is not in itself a useful product, so the quantity recovered has to be reduced as much as possible. One is, therefore, led to consider the balances obtained in each action or stage and to experiment with the concentrations of each constituent of the solution and with the temperature of the solution in order to determine operating conditions for each phase of the process.
In theory, these conditions can be ascertained from equilibrium graphs covering the constituents of the system, i.e., in this case water, sodium chloride, sodium sulphate, and, of course, caustic soda. These graphs show the precipitation of complex salts, made up of sodium chloride, sodium sulphate and soda, such as the triple caustic salt or double salts, during the concentration of aqueous solutions of caustic soda.
It has also been observed that these salts and, particularly the triple salt, are difficult or impossible to decompose in caustic soda solutions, due to secondary phenomena in certain phases of the process.
It is, accordingly, an object of the present invention to provide a process for obtaining caustic soda, pure sodium chloride from an electrolytic cell liquor containing sulphate ions as an impurity.
It is also an object of the present invention to provide such a process which avoids the disadvantages of prior art processes.
Another object of the invention is to employ a multiple-effect technique, thus economizing in heating energy, whereby the number of effects is limited so as not to require installations where capital and maintenance costs would be too high.
A further object of the invention is to optimize the yield of pure sodium chloride, while minimizing extraction of salt containing sulphate without any triple salt and thereby reducing losses of caustic soda.
A still further object is to obtain the caustic soda and sodium chloride in very pure state, employing a process which is highly flexible.
Other objects will be apparent to those skilled in the art from the present description, taken in conjunction with the appended drawing, which is a flow diagram of one process in accordance with the present invention.